Heat resistant resin compositions, articles and method

ABSTRACT

Low viscosity, solventless, thermosetting resin compositions of bismaleimide resin composition and epoxy resins have unique heat stability and special utility as insulation for electric conductors to be used in the 200°-250° C. temperature range.

This is a divisional of application Ser. No. 08/014,046 filed on Feb. 5,1993, now U.S. Pat. No. 5,314,984, which is a continuation ofapplication Ser. No. 07/629,155 filed on Dec. 18, 1990.

FIELD OF THE INVENTION

This invention relates generally to epoxy resin technology and is moreparticularly concerned with novel thermosetting resin compositions oflow viscosities, good electrical properties and high heat resistancecomposed of bismaleimide resin compositions and epoxy resincompositions, and is also concerned with electrical conductors insulatedwith these compositions, and with the method of producing thoseinsulated conductors.

BACKGROUND OF THE INVENTION

The solventless resin compositions of U.S. Pat. No. 4,603,182 havespecial merit as electrical insulation materials because of their lowviscosity and their stability at elevated temperatures, both of whichfavor the use of vacuum-pressure impregnation technique in theproduction of insulated conductors. While those compositions haveconsequently seen extensive use in insulation applications and, in fact,have qualified for use in service at temperatures as high as 220° C.,there persists a well-recognized need for vacuum-pressure impregnatable(VPI) resin compositions for electrical insulating purposes attemperatures up to in the 220°-250° C. range for protracted periods. Thenumerous attempts to produce such heat stable VPI resins have taken avariety of forms, but all have failed for one reason or another tosatisfy the demand. Addition of inorganic oxides and silica, forinstance, has proven to be of very little benefit. Thus, lamellar silicaenhances thermal stability beyond such other inorganic materials butsubstantially increases viscosity of VPI resins, detracting from theirusefulness.

Various heat resistant polymers such as polyimides, fluoro silicones,polyphenylsulfide and the like are useful as films, molded parts andwire enamels but cannot be used at VPI resins for insulating electricalmachinery nor can they be used to impregnate mica paper to makeprepregged mica tapes. Liquid enamels made with high temperaturepolyimide polymers usually contain less than 15% solids and, inaddition, a volatile compound is generated during cure. Polyimides, suchas KAPTON^(T) , find uses in heat resistant electrical insulation asfilms but cannot be used as VPI resins for reasons set out above.

None of the commercially-available heat resistant VPI resins which wehave tested is more heat stable than those of U.S. Pat. No. 4,603,182referenced above.

SUMMARY OF THE INVENTION

By virtue of the present invention, which is based upon our surprisingdiscovery set forth below, it is now possible for the first time to meetto and satisfy the aforesaid demand. Thus, resin compositions of thisinvention possess undiminished the important properties of prior art VPIresins and yet have significantly greater heat resistance than any ofthem. Moreover, these new resin compositions are easily and economicallyproduced and used to provide insulated conductors for service in motorsand generators operating in the 220°-250° temperature range.

In making this invention we discovered that resins which have excellentheat resistance, but are solids at 25° C., can be used under certaincircumstances to produce low temperature solventless VPI compositions.Specifically, we found that the heat stability of the epoxy resincompositions of the -182 patent can be increased enough to meet thespecial needs set out above by adding a bismaleimide resin composition.Vinyl toluene or equivalent aryl vinyl monomer reactive diluent servesas the medium, the epoxy resin composition and the bismaleimide resincomposition both being soluble therein to provide a clear solution.

Briefly stated, this invention in its composition of matter aspect is athermosetting resin having low viscosity at 25° C. and unique thermalstability which consists essentially of a bismaleimide resincomposition, a reactive diluent, and an epoxy resin compositionconsisting essentially of 1, 2 epoxy resin having at least two epoxidegroups per molecule, a small but effective amount of a phenolicaccelerator and a labile halogen-free catalytic hardener.

In its article of manufacture aspect this invention, likewise brieflydescribed, comprises an elongated conductor coated with a thermosettingresin composition as described above or, in the alternative, such aconductor wrapped with a tape impregnated with the thermosetting resincomposition of this invention.

Finally, the method of this invention, as a matter of generaldefinition, comprises the steps of providing an elongated conductor,covering the conductor with a coat of resin composition of thisinvention as described above and then thermally curing the thermosettingresin composition in situ on the conductor. Alternatively, as alsoindicated above, the method may comprise the steps of providing anelongated conductor and providing a tape for wrapping the conductorimpregnating the tape with a resin composition of this inventionwrapping the impregnated tape on the conductor and then thermally curingthe resin composition in the tape in situ on the conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view of an armature winding barwrapped with ground insulation mica paper coated and impregnated orprepregged mica tape in accordance with this invention; and

FIG. 2 is an enlarged, fragmentary, sectional view of an electricalconductor like that of FIG. 1 provided with vacuum-impregnatedinsulation in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While one has a wide latitude of choice as to both the epoxy resincomposition and the bismaleimide resin composition in practicing thisinvention, our present preference is to use as the epoxy resincomposition that hereinafter identified Resin A, and to use as thebismaleimide resin composition that which is commercially available as aproduct of the Shell Chemical Company under the trade name "Compimide353". The latter is a hot melt-type eutectic mixture of bismaleimideresins based on methylene dianiline and aliphatic diamines and has amelting point of 68° to 129° C. This particular maleimide resincomposition is favored for this purpose because of its high solubilityalong with epoxy resin compositions at 70° to 100° C. in aryl vinylmonomer.

Resin A is a clear resin having viscosity of 470 centipoises (cps) at25° C. and gel time of 8.0 minutes at 171° C. The method of itsproduction and additional description of Resin A are set forth inExample 29 of the aforesaid -182 patent which is incorporated herein byreference.

Resin A and Compimide 353 are used in approximately equal proportionsand vinyl toluene is added in an amount such that the proportion ofvinyl toluene in the total resin composition of this invention isbetween approximately 25% and 40% for VPI applications and can containas low as 5% aryl vinyl monomer for a prepregged mica tape. Thesematerials are mixed and stored at approximately 80° C. until the mixtureis substantially uniform throughout.

In accordance with this invention other bismaleimide resin compositionscan be used and likewise the present new advantages and results can beobtained through the use of epoxy resin compositions other than Resin A.Thus, eutectic blends of an aliphatic bismaleimide and an aromaticbismaleimide, which as eutectics have melting points in the range ofabout 60° C. to about 130° C. and consequently are soluble with Resin Aand the like in vinyl toluene or other such aryl vinyl monomer, arereasonable alternatives to the preferred compositions stated above.

Reactive diluents other than vinyl toluene are useful in the practice ofthis invention as previously indicated. While we prefer the latter,styrene, alpha-methyl styrene, an isomer or a mixture of isomers ofvinyl toluene, of t-butyl styrene, of divinyl benzene, and ofdiisopropenyl benzene and mixtures thereof can be used. As used herein"vinyl toluene" refers to a mixture of the meta-and para-methyl styreneisomers, but a single isomer such as para-methyl styrene may be used,and similarly t-butyl styrene refers to para-t-butyl styrene or amixture of the ortho, meta and para isomers. Divinyl benzene anddiisoprophenyl benzene also refer to one isomer or mixtures of theortho, meta and para isomers. Further, divinyl benzene may also containa substantial quantity of ethyl vinyl benzene.

In preparing the present thermosetting resin compositions in accordancewith our presently preferred mode, Resin A is compounded with vinyltoluene as described in Example 29 of the -182 patent. Compimide 353 isadded in approximately equal portion to Resin A and vinyl toluene isadded in amount to bring the total in the mixture to about 40%. Thus,our preference is for the upper end of the 25 to 40% range of vinyltoluene content of the thermosetting resin composition and for the lowerend of the range of bismaleimide resin composition content at about 34%,the upper end of that range being about 43%. The mixture as thuscompounded as stirred at approximately 80° C. until it is substantiallyuniform throughout.

As thus produced, the thermosetting resin composition of this inventioncan be employed in providing insulating coatings and coverings forelectrical conductors, being applied by spraying, dipping or brushingthem on a conductor surfaces in requisite thickness. The coatings arethen cured to hard, tough solids by subjecting them to temperature of160° C. for a suitable time, such as up to 15 hours. The thermosettingresin containing as low as 5% aryl vinyl monomer and having a higherviscosity than used in VPI processing may be used to impregnate a micacontaining tape to form a prepregged or resin-rich mica tape which iswrapped around a conductor and then cured under heat and pressure toconsolidate the insulation. Alternatively, these resins can be used invacuum-pressure impregnation applications to fill glass fabric, micapaper or mica flake tapes or the like which have been wrapped around theconductors and assembled in the winding and then curing the impregnatedcomposite resin material in situ by a heat curing operation as describedabove.

The resulting sheets or tapes can be wound by hand or by machine asground or other insulation on electrical components such as a conductorbar shown in FIG. 1 of the drawings, accompanying this specification.Thus, a typical conductor bar 1 having a plurality of conductor turns 2insulated from each other in the usual manner has arrays of conductorsseparated by strand separators 4. Wrapped around the armature windingbar is ground insulation 5 of a plurality of layers of mica paper tape6, coated and impregnated with the resin composition of this invention.The wrapped conductors can be assembled in the machines and thenvacuum-pressure impregnated with the resins of this invention followedby draining the excess resin and baking to cure the resin. Anotherprocess is to make a mica tape containing the solventless resin of thisinvention then wrapping the prepregged tape around the conductor. Inpreparing such an insulated conductor bar, the entire assembly iscovered with a sacrifice tape and placed in a pressure tank andevacuated. There is no need in this process to remove solvents from thepresent resin compositions, the only purpose of the evacuation being toremove entrapped air. After vacuum treatment, molten bitumen, or otherheat transmitting fluid is introduced into the tank under pressure tocure the resin in well known manner. Upon completion of the curing step,the insulated conductor is removed from the bath, cooled and thesacrifice tape is removed.

An enlarged, fragmentary, sectional view of an electrical conductor 7 isprovided in FIG. 2, the conductor being provided with vacuum-impregnatedinsulation 8 according to this invention. Thus, there are two layers ofmica paper 9 with reinforcement or backing material 10 and a small space11 between these layers and another layer 12 between the inner tapelayer and conductor 7. Spaces 11 and 12 and the tape layers themselvesare filled by the resinous composition as depicted with cross-hatchingindicated by reference character 13. Such complete filling of thisinsulating structure and the void free nature of the conductor coveringare attributable to the low viscosity of the novel composition of thisinvention and to the fact that it contains no solvent to be removedduring the curing operation.

Departures from our presently preferred practice described in detailabove are contemplated and covered generally and specifically by theappended claims as the new results and advantages of this invention canbe obtained consistently through the use of other bismaleimide resincompositions and epoxy resin compositions in similar thermosetting resincompositions. Thus, in regard to bismaleimide resin compositions, asthose skilled in the art recognize, they are oligomers or bisimidemonomers usually derived from maleic anhydride and aromatic diaminessuch as methylene diahiline. Aromatic amines and allyl phenols have beenused as co-curing materials for the bismaleimide resins. An allyphenolused with bismaleimide resin is o-o'-diallyl bisphenol A. There are manysuch resins and a large number of those are suitable for use inaccordance with this invention but again, Compimide 353 offers thespecial advantage of high solubility in epoxy resins to formepoxy-bismaleimide resin composition solutions.

Epoxy resins which are suitable for use in accordance with thisinvention include those which are disclosed and claimed in the aforesaidU.S. Pat. No. 4,603,182. In fact, they are detailed as to preparationand composition in the working examples set forth in that patent. Thoseportions of that patent pertaining to the epoxy resin compositions ofthe invention therein claimed are hereby incorporated in this disclosureby reference.

Those skilled in the art will gain a further and better understanding ofthe present invention and the new results and advantages thereof in thefollowing illustrative, but not limiting, examples of the practice ofthis invention as it has actually been carried out experimentally.

EXAMPLE 1

A clear amber, 4,500 Centipoises viscosity at 25° C. resin was obtainedby heating at 70° C. and stirring 50.00 parts-by-weight (pbw) of ResinA, 16.7 pbw vinyl toluene, and 50.0 pbw of Compimide 353 resin. Thisresin contains 25 percentage by weight vinyl toluene, identical to the25% by weight of Resin A. The weight losses at 260°, 280° and 300° C.were significantly lower with the Compimide 353 modified resin thanResin A when tested together and under identical test conditions. Weightloss measurements were made on 10-gram, 2.1-inch diameter discs whichwere aged in forced air circulating ovens. The Resin A control samplewas tested together with the new resin, therefore, comparisons in thisexperiment and those following below were made of materials that sawidentical time and temperature test conditions.

    ______________________________________                                        % Weight Loss                                                                                Resin A                                                                              Example 1                                               ______________________________________                                        21 days @ 260° C.                                                                       8.68     4.81                                                21 days @ 280° C.                                                                       14.41    7.41                                                10 days @ 300° C.                                                                       25.32    8.72                                                ______________________________________                                    

In addition, while the aged Resin A samples showed extensive surfacecracks and embrittlement, the Example 1 resin samples were tough, glossysolids with no surface cracks or other signs of thermal deterioration.

EXAMPLE 2

A clear, 2,500 Centipoises viscosity @25° C. resin was made by stirringand heating at 70° C. 50.0 pbw of Resin A, 20.7 pbw vinyl toluene and50.0 pbw Compimide 353. The resin was significantly more heat stablethan Resin A.

    ______________________________________                                        % Weight Loss                                                                                Resin A                                                                              Example 2                                               ______________________________________                                        21 days @ 260° C.                                                                       8.68     4.57                                                21 days @ 280° C.                                                                       14.41    6.91                                                10 days @ 300° C.                                                                       25.32    8.15                                                ______________________________________                                    

The heat aged Example 2 resin samples had no cracks or other signs ofthermal deterioration compared to the extensive surface cracking andembrittlement of Resin A during the 280° and 300° C. aging.

EXAMPLE 3

A clear, 1,800 Centipoises @25° C. resin was made from Resin A (50.0pbw), vinyl toluene (25.0 pbw) and Compimide 353 resin (50.0 pbw) byheating and stirring at 70° C. The resin was significantly more heatstable than Resin A:

    ______________________________________                                        % Weight Loss                                                                                Resin A                                                                              Example 3                                               ______________________________________                                        21 days @ 260° C.                                                                       8.68     4.39                                                21 days @ 280° C.                                                                       14.41    6.91                                                10 days @ 300° C.                                                                       25.32    8.15                                                ______________________________________                                    

Unlike the Resin A, the Example 3 resin samples remained tough solidswith no formation of surface cracks after the heat aging at 280° and300° C.

EXAMPLE 4

A 1,000 Centipoises @25° C. resin was made from Resin A (50.0 pbw),vinyl toluene (29.6 pbw) and Compimide 353 resin (50.0 pbw) by heatingand stirring at 70° C. The heat stability of this resin wassignificantly better than that of Resin A.

    ______________________________________                                        % Weight Loss                                                                                Resin A                                                                              Example 4                                               ______________________________________                                        21 days @ 260° C.                                                                       8.68     4.34                                                21 days @ 280° C.                                                                       14.41    6.84                                                10 days @ 300° C.                                                                       25.32    8.08                                                ______________________________________                                    

As in Examples 1 to 3, the Example 4 resin remained a tough solid withno formation of surface cracks or other signs of thermal deterioration.

EXAMPLE 5

A 320 Centipoises @25° C. resin was made from Resin A (50.0 pbw), vinyltoluene (34.6 pbw), and Compimide 353 resin (50.0 pbw). The resin wassignificantly more heat stable than Resin A.

    ______________________________________                                        % Weight Loss                                                                                Resin A                                                                              Example 5                                               ______________________________________                                        14 days @ 260° C.                                                                       7.14     3.59                                                14 days @ 280° C.                                                                       12.55    5.85                                                 7 days @ 300° C.                                                                       15.15    6.94                                                ______________________________________                                    

The extensive cracking and embrittlement of the Resin A samples ages at280° and 300° C. were not present in the Example 5 resin samples.

EXAMPLE 6

A 200 Centipoises @25° C. viscosity resin was made from Resin A (50.0pbw), vinyl toluene (40.0 pbw), and Compimide 353 resin (50.0 pbw). Theresin was significantly more heat stable than Resin A.

    ______________________________________                                        % Weight Loss                                                                                Resin A                                                                              Example 6                                               ______________________________________                                        14 days @ 260° C.                                                                       7.14     3.44                                                14 days @ 280° C.                                                                       12.55    5.67                                                 7 days @ 300° C.                                                                       15.15    6.76                                                ______________________________________                                    

Unlike the Resin A, no cracks or embrittlement occurred in the Example 6resin samples.

EXAMPLE 7

A 100 Centipoises @25° C. resin was made by heating at 80° C. andstirring Resin A (50.0 pbw), vinyl toluene (45.8 pbw), and Compimide 353resin (50.0 pbw). This resin was significantly more heat stable thanResin A.

    ______________________________________                                        % Weight Loss                                                                                Resin A                                                                              Example 7                                               ______________________________________                                        14 days @ 260° C.                                                                       7.14     3.33                                                14 days @ 280° C.                                                                       12.55    5.52                                                 7 days @ 300° C.                                                                       15.15    6.64                                                ______________________________________                                    

The cracking and embrittlement of Resin A aged at 280° and 300° C. werenot present in the Example 7 resin samples.

Those skilled in the art will understand that there are possibleapplications of this new chemistry in addition to vacuum-pressureimpregnation resins and resins for making electrical insulationprepregs, and that those applications include thermosetting resins forproduction of heat resistant resin-glass laminations, coating moldingand potting compounds, adhesives, tooling composites based on glasscarbon fibers and other reinforcements and the like.

In this specification and the appended claims, where percentage,proportion or ratio is state, reference is to the weight basis unlessotherwise specified.

We claim:
 1. An article of manufacture comprising an elongatedconductor, a tape wrapped on the conductor, said tape being impregnatedwith a solventless vacuum-pressure impregnatable thermosetting resincomposition having viscosity less than 4,500 cps at 25° C. and uniquethermal stability consisting essentially of a bismaleimide resincomposition, vinyl toluene, an epoxy resin composition consistingessentially of a nitrogen-free 1, 2 epoxy resin having at least twoepoxide groups per molecule, and a small but effective amount of aphenolic accelerator and a labile halogen-free catalytic hardener.
 2. Anarticle of manufacture comprising an elongated conductor coated with asolventless vacuum-pressure impregnatable thermosetting resincomposition having viscosity less than 4,500 cps at 25° C. and uniquethermal stability consisting essentially of a bismaleimide resincomposition, an aryl vinyl monomer, an epoxy resin compositionconsisting essentially of a nitrogen-free 1,2 epoxy resin having atleast two epoxide groups per molecule, and a small but effective amountof a phenolic accelerator and a labile halogen-free catalytic hardener.3. The method of producing an insulated elongated conductor whichcomprises the steps of providing an elongated conductor, covering theconductor with a coat of a solventless vacuum-pressure impregnatablethermosetting resin composition having viscosity less than 4,500 cps at25° C. and unique thermal stability consisting essentially of abismaleimide resin composition, aryl vinyl monomer, and an epoxy resincomposition consisting essentially of a nitrogen-free 1,2 epoxy resinhaving at least two epoxide groups per molecule and a small buteffective amount of a phenolic accelerator and a labile halogen-freecatalytic hardener, and then thermally curing the resin composition insitu on the conductor.
 4. The method of producing an insulated elongatedconductor which comprises the steps of providing an elongated conductor,providing a tape for wrapping on the conductor, impregnating the tapewith a solventless vacuum-pressure impregnatable thermosetting resincomposition having viscosity less than 4,500 cps at 25° C. and uniquethermal stability consisting essentially of bismaleimide resincomposition, vinyl toluene, and an epoxy resin composition consistingessentially of a nitrogen-free 1, 2 epoxy resin having at least twoepoxide groups per molecule and a small but effective amount of aphenolic accelerator and a labile halogen-free catalytic hardener, thenwrapping the impregnated tape on the elongated conductor, and finallythermally curing the resin composition in situ on the tape-wrappedconductor.
 5. The method of producing an insulated conductor whichcomprises the steps of producing an elongated conductor, wrapping aporous tape on the elongated conductor, impregnating the tape on thesaid conductor with a solventless vacuum-pressure impregnatablethermosetting resin composition having viscosity less than 4,500 cps at25° C. and unique thermal stability consisting essentially of abismaleimide resin composition, vinyl toluene and an epoxy resincomposition consisting essentially of a nitrogen-free 1, 2 epoxy resinhaving at least two epoxide groups per molecule and a small buteffective amount of a phenolic accelerator and a labile halogen-freecatalytic hardener, and then thermally curing the resin composition insitu on the tape on the conductor.